Lasers move droplets

November 3/10, 2004

Labs-on-a-chip promise inexpensive and
portable biological and chemical analysis. Key to making the tiny labs
work is finding ways to move and mix minuscule amounts of substances.

Scientists have devised several methods of manipulating droplets,
including moving them with light and electric fields, and using heat to
change surface tension. Light, or laser tweezers, methods require droplets
to be contained in membranes. Electric field and heat methods require
surfaces with patterned electrodes.

Researchers from SRI International have demonstrated a relatively
simple method of manipulating droplets that takes advantage of the thermal
Marangoni effect: when a droplet is warmer along one side than the other,
it shifts toward the cooler side to minimize surface energy. Heat from
a laser changes droplet surface tension, which causes droplets to move
away from the laser beam.

The method could eventually be used for high throughput biological
and chemical screening, and fluidic, chemical and biological experiments
that use minuscule amounts of samples and chemicals, according to the
researchers.

The researchers demonstrated the method using drops of dyed water
on a film of oil on a Petri dish. They were able to move individual droplets
ranging from 30 microns to 1,500 microns in diameter as fast as 3 millimeters
per second. A micron is one-thousandth of a millimeter. The method can
also quickly mix droplets. The researchers mixed a 187-microliter droplet
of dye and a 182-microliter droplet of black ink in 33 millionths of a
second.

Because the method requires no electrical circuitry, devices can
be made cheaply and droplets can move anywhere on the surface, making
labs-on-a-chip easy to reconfigure.

The method could be used practically within a couple of years,
according to the researchers. The work appeared in the September 27, 2004
issue of Applied Physics Letters.